1. Field
Embodiments of the present invention relate to a color image forming apparatus to enable color registration correction.
2. Description of the Related Art
Generally, in a color image forming apparatus, light is irradiated to a photoconductive drum that has been charged with a predetermined electric potential, to form an electrostatic latent image. After developing the electrostatic latent image using a predetermined color of developer, the resulting developer image is transferred and fused to paper, completing formation of a color image.
Of a variety of color image forming apparatuses, recently, a tandem type color image forming apparatus has been frequently used, in which a plurality of developing devices and photoconductive drums are provided on a per color basis to form an image in a single-pass manner. The tandem type color image forming apparatus includes four photoconductive drums, developing devices, exposure devices and a transfer belt. The four photoconductive drums correspond to yellow, magenta, cyan and black developers. Each exposure device scans light to the corresponding photoconductive drum to form an electrostatic latent image according to a user input.
The developing devices supply different colors of developers to electrostatic latent images formed by the exposure devices to form different colors of developer images. The developer images formed on the respective photoconductive drums are sequentially transferred to and overlap one above another on the transfer belt and in turn, the resulting color image is transferred to paper.
To print an accurate color image, it may be necessary for all of the different colors of developer images to have the same transfer beginning position and the same transfer ending position from the respective photoconductive drums to the transfer belt. Consequently, to accurately realize a color image, it may be important to accurately coincide an exposure time of the exposure devices for the respective photoconductive drums in consideration of a traveling velocity of the transfer belt. Here, accurately setting the exposure time is called color registration.
However, even if the exposure time is initially accurately set, mis-registration may occur as printing proceeds. This is because a driving roller used to travel the transfer belt may expand due to heat generated during printing. Specifically, if the driving roller expands thus changing a diameter thereof, the traveling velocity of the transfer belt may be changed despite constant revolutions per minute of the driving roller and therefore, it may be necessary to adjust the exposure time. Always accurately realizing a desired color image by dynamically controlling the exposure time is called mis-registration correction, i.e. color registration correction.
In a conventional color registration correction method, after exposure devices form predetermined patterns on photoconductive drums and in turn, the predetermined patterns are developed and transferred to a transfer belt, a photo-sensor including a light emitting element and a light receiving element detects a degree of mis-registration between the respective color patterns, allowing an exposure time of the respective color patterns to be corrected based on the degree of mis-registration.
However, since the corrected exposure time, which is acquired via a single initial calculation, is applied to the overall printing operation, system stability may be essential for perfect color registration correction. However, in an actual system, respective color images may be printed at different positions of respective recording media. This causes mis-registration corresponding to a difference between printing positions of the recording media.